The current and planned research examines question of fundamental importance in the field of medical and veterinary parasitology. The research is aimed at determining the relationship between alterations in the structure and function of the immune system and the progression of the disease process in African trypanosomiasis. Primarily, the underlying cellular mechanisms of the immunosuppression associated with experimental chronic African trypanosomiasis are being studied. Specific B lymphocyte, T lymphocyte and macrophage cell functions are being assessed in mice at various stages of infection. Conclusions are being made concerning the level of immunosuppression, the nature of immunosuppression, the cause of immunosuppression and the significance of the immunosuppression in terms of the host-parasite relationship. When the basic mechanisms of induction, and the significance, of such immune system alterations are clear, then rational approaches to specific forms of immunotherapy or prophylaxis may be taken. Ultimately, blockage or elimination of immunological defects in trypanosome infections of humans or domestic animals may result in resolution of the pathology associated with the disease as well as of the relapsing nature of the infections.